It seems to me that the first method
ClassName.myNumber
makes significantly more sense. Instantiating every time you want to change/get myNumber requires a lot more work. Also, its much easier to pick out that its a static field.

In my personal opinion, static methods and fields should be accessed by MyClass.myMethodOrField, as it clearly states that the method/field is static (as long as you conform to the "object names start with lowercase" standard). Furthermore, if you access it from an object, you could come back later and forget that it's static, modify it's value, and then end up with nasty little bugs.